Films produced from polymers such as polyolefins, polystyrene, poly(vinyl chloride), polyesters and the like, are used frequently for the manufacture of labels for plastic beverage or food containers. Because these labels are often recycled, it is desirable that the label materials are compatible with the recycling process streams and not cause excessive contamination of those streams. For example, in most recycling operations, the bottle or container is the primary object of recycle while the label, because of its printing inks and glues, generally is considered a “contaminant”. As a result, the label is usually isolated and removed. For example, poly(ethylene terephthalate) (“PET”) bottles typically use non-shrinking, roll-fed polypropylene (“PP”) labels. In a typical recycling operation, the PET bottle polymer is collected and cleaned for reuse, while the polypropylene label is separated and discarded. The separation of these two materials can be easily accomplished by a sink/float process in which the flaked bottle and the label are suspended in water and separated on the basis of their densities. Sink/float processes are particularly efficient for the separation of PET and PP because of the large differences in the densities of these polymers. For example, the density of the water used in most recycle operations is about 1.03 to 1.05 g/cc because of the presence of contaminants, caustic (sodium hydroxide), and solids in the water. PET, with a density of around 1.35 g/cc, will sink to the bottom during recycling processing. Polypropylene, however, has a density of about 0.90 g/cc and will float to the top where it can be skimmed off. This separation method has made the recycle of PET bottles with PP labels efficient and commercially successful.
By contrast to non-shrinking PP labels, most shrink labels made from polymers such as, for example, polyester, polystyrene, and poly(vinyl chloride), have a high densities and can not be separated from other higher density polymers, such as PET, in a sink/float process. For example, the typical density is about 1.30 g/cc for polyester shrink labels, about 1.05 g/cc for polystyrene labels, and about 1.33 g/cc for PVC labels. If these labels are not removed prior to the sink/float step by some other means such as, for example, by air elutriation or by manually tearing them off of the bottle, they will sink with the PET and eventually cause color and haze contamination. For labels made from PVC, this contamination is particularly undesirable as PVC emits corrosive hydrochloric acid at PET processing temperatures. Polystyrene labels are low enough in density that most of the flakes tend to hang in the sink/float tank, and can be partially separated by filtering the water. The presence of small amounts of polystyrene with recycled PET, however, can cause offgassing and release of hazardous styrene monomer during subsequent PET processing. Polyester shrink labels, by contrast, are usually more compatible with reprocessed PET, but still present contamination problems from printing inks and glues. A polyester label that could be separated by sink/float processes, therefore, would be highly desirable for packaging applications.
One approach for improving the recycle of polyester shrink labels is to mechanically reduce their density below that of water, for example, by foaming or voiding. Foaming is effective for decreasing the density, but the resulting film is difficult to print and lacks desirable aesthetics. Void-containing films, by contrast, are easy to print and have a desirable opaque matte finish. Voids are obtained by incorporating about 5 to about 50 weight % of small organic or inorganic particles or “inclusions” (referred in the art as “voiding” or “cavitation” agents) into a polyester and orienting the polymer by stretching in at least one direction. During stretching, small cavities or voids are formed around the voiding agent. When voids are introduced into polyester films, the resulting void-containing film has a lower density than the non-voided film, becomes opaque, and develops a paper-like surface. This surface also has the advantage of increased printability; that is, the surface is capable of accepting many inks with a substantially greater capacity over a non-voided film. Typical examples of voided films are described in U.S. Pat. Nos. 3,426,754; 3,944,699; 4,138,459; 4,582,752; 4,632,869; 4,770,931; 5,176,954; 5,435,955; 5,843,578; 6,004,664; 6,287,680; 6,500,533; 6,720,085; U.S. Patent Application Publication No.'s 2001-0036545; 2003-0068453; 2003-0165671; 2003-0170427; Japan Patent Application No.'s 61-037827; 63-193822; 2004-181863; European Patent No. 0 581 970 B1, and European Patent Application No. 0 214 859 A2.
Although voided polyester films can be manufactured to have densities below 1 g/cc, these films do not normally retain these lower densities after shrinkage. Density increases of 0.05 to 0.15 g/cc are common under standard shrinkage conditions (e.g. 5 to 10 seconds in a hot air or steam shrink tunnel at 80 to 90° C.). This increase in density or “densification” results from a reduction in the size of the voids during the shrinkage of the polyester film and can continue in recycling processes, which often employ hot water for grinding or washing the polymer. For example, many recycling processes involve an initial wet or dry grinding step in which the bottles and labels are ground into smaller flakes, followed by a flake-washing step where the combined mix of ground up PET polymer and label are washed for 10 to 15 minutes in a caustic bath at 85° C. (the caustic bath typically consists of 1 to 2 wt % sodium hydroxide in water). In this flake washing process, the film/flake can continue to shrink and tends to absorb water, which further increases the density of the film by filling up some of the voids with water. This shrinkage and absorption of water can increase the density of the polyester film by as much as 0.15 to 0.30 g/cc above the initial densities of the unshrunk film and cause the polyester label material to sink with the PET bottle polymer.
One option for remedying the above problem is to increase the number of voids in the initial film to impart a lower starting density in order to compensate for shrinkage-induced densification. This remedy may be accomplished by adding more voiding agent to the film. Increasing the level of voiding agent, however, usually makes the film rough, easily teared, and unacceptably brittle. Furthermore, the increase in density upon shrinkage, generally, is proportional to the amount of voiding agent present. Thus, although the starting film density is reduced significantly with increased voiding agent, the increase in the density after shrinking and during recycling also is greater and there is little overall net benefit. Simply increasing the level of voiding agent, therefore, is not a fully satisfactory approach for most applications.
Another option is to heatset the film. Heatsetting polyesters traditionally involves heating the film at temperatures from about 180 to 200° C. after stretching while constraining the film in place. It is commonly used in PET and non-shrinking, microvoided PET films where dimensional stability is required. While this heatsetting does keep the film from increasing its density during the recycle process, it also prevents shrinking and, thus, is unacceptable for use in a shrink film.
In view of the above shortcomings, there is a need for a void-containing polyester shrink film that will simultaneously maintain a high degree of shrinkage while maintaining a low density during recycle processing. It is also desirable that this film maintain adequate smoothness, tear resistance and aesthetics. Such a film would have utility in the beverage and food packaging industry for the production of recycle-friendly, void-containing shrink labels.